It’s possible that you have some very large surfaces like the floor that are simply too large–the lightmap is an actual image so it has to cover the entire surface so if it’s very large it would need a very high resolution lightmap so it can happen where the object is too large to get enough detail even with a high resolution lightmap. Also, I’m not sure if you can control it in Fabric Engine but if it has bad lightmap UV’s then increasing the lightmap resolution can’t fix it.
It will certainly speed up exporting if your objects are already converted to meshes beforehand